Speakon Connector

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Hello, can you connect a 1/4 " jack into the centre of a speakon female connector?

No, that's not how Speakon connectors work. You'll need some kind of adapter, because plugging it in the way you mentioned would just connect both conductors of the 1/4" to the center conductor of the Speakon. It wouldn't really hurt anything, it just wouldn't work.